The current work investigates the role of boron carbide (B4C) reinforcements on the material flow, microstructure and mechanical properties of dissimilar friction stir welded Al5083-6061 aluminum alloys. Four different weld composites were prepared with varying B4C composition (0–4%). The fine B4C particles were added at the adjoining edges by a novel reinforcing technique along the weld line. Optical and scanning electron microscopes were used to study the B4C distribution, dissolution, bonding and fracture regions. The B4C pinned the grain boundaries and the fine grain structure is obtained by preventing the grain growth during the recrystallization process. Mechanism of microstructure evolution in the stir zone with B4C is attributed to dynamic recovery, dynamic recrystallization and particle stimulated nucleation. The highest hardness and strength were obtained in the joint processed with 2% B4C reinforcements. The tensile test results reveal that the weld composite exhibits higher strength not only by decrease in grain size but also due to grain refinement strengthening and orowan strengthening mechanisms which are occurred due to addition of B4C particles.